The CERN teams recently accomplished a real scientific tour de force with a measurement of diabolical precision: thanks to the famous Large Hadron Collider (LHC), they recalculated the mass of the W boson, one of the fundamental particles that govern the behavior of matter. This success definitively buries an anomaly that threatened to collapse an entire section of particle physics — for better or for worse.
Today, to describe the world around us at the smallest of scales, physicists rely largely on what is called the Standard Model of particle physics. It is thanks to this extremely solid theoretical framework that we can classify all known elementary particles into different groupsand explain how three of the four fundamental forces of nature work, the exception being gravity—a detail that will be important later.
There are two main families. On one side, we have the fermionswho are the elementary bricks of matter. They are themselves divided into baryons (protons, neutrons, etc.) and leptons (electrons, neutrinos, etc.) which all share a common characteristic: in accordance with the Pauli exclusion principletwo fermions cannot occupy the same space at the same time, and it is this property that makes matter “solid”.
On the other hand, there are the bosonswhich can be considered as force vectors. Two of them are particularly famous. The first is the unmissable photonthe vector of the electromagnetic force. The second is the boson de Higgsthe famous “ God particle ” which has been the subject of a fascinating scientific quest.
Particle mass, the key to the “ new physics »
But these are not the only ones; there are also other bosons that are less well-known but just as important for fundamental physics. One example is gluons, which govern what is called the strong interaction — the fundamental force that gives atoms their coherence. There are also Z and W bosonswhose dynamics is at the origin of the weak interaction. This is another fundamental force that is responsible for very important phenomena, such as radioactive decay.
Since the introduction of the Standard Model, physicists have been trying to measure the mass of each member of this nanometric menagerie. And it is not just for the pleasure of building a catalog. Since mass is a fundamental property of nature, knowing the mass of the fundamental bricks of the Universe could lead to immense progress in understanding all the phenomena that take place there.
Moreover, these measurements are also a way of testing the limits of the Standard Model. Because even though it has proven exceptionally robust so far, physicists know full well that it is at best incomplete. Indeed, this theoretical framework does not include any way of explaining gravitational force, the fourth fundamental force of nature that is nevertheless so well explained by Einstein’s famous theory of relativity.
To reconcile the two into a single unified theory and open the door to a ” new physics » revolutionary, describing the whole of observable reality, we must therefore begin by identifying these gaps. These mass measures are a central element of this approachbecause the slightest deviation between reality and theory would be an extremely promising indicator.
The W boson, as promising as it is recalcitrant
So far, these gigantic efforts have never produced conclusive results. For decades, relativity has been taking the battering rams of physicists without flinching, and the standard model also continues to prove unshakeable.
But many researchers believe that the key to the problem may lie in this famous W boson. Indeed, it is one of the very few particles in the Standard Model whose mass can be calculated, but also experimentally verified. This makes it particularly promising when it comes to testing the limits of the model.
The problem is that Measuring the mass of the W boson is a notoriously difficult exercise. which involves a significant amount of uncertainty.
To measure a particle’s mass, physicists have to use a crude method: smashing it into a particle accelerator, then measuring the cumulative energy of the individual sub-particles produced by the collision. It’s a bit like throwing a LEGO building against a wall, then measuring the individual pieces that lie on the floor.
However, the W boson is not very well suited to this. It is a notoriously unstable particle that tends to disintegrate spontaneously, producing a muonbut also a neutrinothe famous ” ghost particle “These objects almost never interact with ordinary matter, and so they tend to escape the detector without leaving any trace. To use the example above, it’s as if half the LEGO pieces flew out the window on impact; a crucial part of the information about the original object is missing. It is therefore very difficult to obtain precise data on the W boson.
A potentially revolutionary anomaly…
But the last time a lab did it, the W boson came close to shattering the status quo. At the time, researchers at Fermi National Accelerator Laboratory (FNAL) published a remarkable studywho has concluded that the mass of the W boson was larger than expected.
This result had the effect of a bomb in the scientific sphere, and for good reason: it looked very much like a real breach into which physicists could finally rush to reconcile relativity and the standard model, in order to open the door to the ” new physics » long awaited.
Until now, the gigantic efforts of physicists had never produced conclusive results. For decades, relativity has been taking the battering rams of specialists without flinching, and the standard model also continues to prove unshakeable; but this time, there were real reasons to hope for real progress.
…which falls like a soufflé
But that was without counting on the physicists from CERN, who came to play spoilsport. This new twist is the fruit of the CMS experiment, a program based on the LHC — the most powerful particle accelerator on the planet.
For about ten years, its operators have analyzed hundreds of millions of muons produced by the decay of W bosons. Recently, they finally obtained enough experimental data to compare them to different simulations where they assigned different masses to the boson, in order to verify which model was the most consistent. Thanks to this methodology, they were able to calculate the mass of the W boson with unprecedented precision.
And the result is as remarkable as it is disappointing: everything indicates that the mass of the W boson corresponds perfectly to the theory formalized in the standard model. According to the authors, the many independent physicists who have looked at these results have not detected any methodological or mathematical problems. In other words, the FNAL measurement was probably an anomaly, and CERN has just given a cold shower to those who saw it as the beginning of the ” new physics ».
« It would probably have been better for the community to come up with something totally different from the standard model, as that would have been exciting for the future of our discipline. “regrets Elisabetta Manca, particle physicist at the University of California quoted by Nature.
The Quest for the Theory of Everything Continues
But this is not an end in itself, quite the contrary. Even if the scientific community seems convinced by CERN’s results, no one can yet explain the source of the anomaly detected by FNAL, whose methodology was nevertheless very solid. And to clear up this matter, it will be imperative to find what is wrong. The good news is that CERN’s long experience has also led to the emergence of new software and mathematical tools that will allow the investigation to be pushed even further.
In any case, specialists are accustomed to this kind of disappointment. Cases of this kind are legion in particle physics; no later than this summer, new measurements of the famous Higgs boson have also doused hopes of an imminent reconciliation between the standard model and relativity.
They know full well that the road will be long and difficult, but these disappointments have never diminished the legendary tenacity of physicists – and this new episode will not stop them either. They will continue to mistreat the standard model for many years, while their colleagues do the same with relativity. And with a lot of time and a little luck, these efforts may end up leading us to the missing pieces of the puzzle of the famous ” Theory of Everything ” continued by Einstein and his successors.
Source: www.journaldugeek.com
